Statistics show foreign-born residents commit fewer crimes, but backers of an illegal immigrant law say public safety is their key motivation.

By many measures, Arizona has become safer since illegal immigrants began pouring into the state in the 1990s.

Crime has dropped all across the country since then, but the decrease has been as fast or faster in Arizona. The rate of property crimes in the state, for example, has plummeted 43% since 1995, compared with 30% nationwide.

That's no surprise to those who study immigration — both sides, whether for or against increased immigration, agree that immigrants tend to commit fewer crimes than native-born Americans.

Nonetheless, authors of a controversial new law against illegal immigration here have long cited the need to fight crime as a key reason behind SB 1070, or the Support Our Law Enforcement and Safe Neighborhoods Act. The law makes it a state crime to lack immigration papers and requires police to determine whether people they stop are in the country illegally.

Backers repeatedly have cited the killing of two Phoenix police officers by illegal immigrants since 2007, or the recent slaying of a cattle rancher near the Mexican border by a drug smuggler. State Rep. John Kavanagh, a co-sponsor of the law, said of illegal immigrants, "They bring a lot of crime with them." On Friday, that argument got more momentum when a deputy sheriff was wounded in a gun battle with men suspected of being drug smugglers from Mexico.

The SB 1070 proponents also point to incarceration rates as a sign that illegal immigrants may contribute excessively to crime in Arizona.

Phoenix Police Chief Jack Harris said about 10% of his department's arrests are illegal immigrants — a number close to the estimated percentage of undocumented migrants in the local population — but the Maricopa County sheriff's office, which runs the jail for Phoenix and surrounding cities, said 20% of its inmates are illegal immigrants. Fifteen percent of state prisoners are illegal immigrants.

The bill's proponents contend that criminals in Mexico are increasingly heading north through Arizona.

"A large portion of [illegal immigrants] are coming here seeking a life and, quite frankly, fleeing the violence in Mexico," said Brian Livingston, executive director of the Arizona Police Assn., who added he was persuaded to back SB 1070 by calls from a Latina complaining that no one arrested illegal immigrant gang members in her neighborhood. "Amongst those people are criminal elements who prey on those people," he said.

"Those are what we're targeting with this bill," Livingston said. "We're targeting the smugglers who prey on the good element of the Mexican population."

Phoenix has become a hub of human trafficking, and now it has kidnapping numbers that rival cities in Mexico because of smugglers who hold illegal immigrants hostage in drop houses in the city. The city's crime rates are comparable with those of other big cities, but the presence of well-armed trafficking groups colors the picture.

"It may be safer in Beirut than Phoenix," said Mark Spencer of the Phoenix Law Enforcement Assn., citing a report that some illegal immigrants were selling grenades on the black market.

Harris, the Phoenix police chief, who opposes SB 1070, said proponents of cracking down on illegal immigrants vastly overstated that population's criminality.

"Saying that if you get rid of the illegal immigrants, you'll get rid of 80% of the crime, which I've heard, that's not true," he said, dismissing the rhetoric as political opportunism. "All you have to do in Arizona is come out with anything that's anti-immigrant and you will be in good shape in the polls."

What most in law enforcement here do agree on is that the victims of crime by illegal immigrants tend to be other immigrants. Community activists argue that the new law will make it worse for law-abiding immigrants because few immigrants, whether documented or not, will want to deal with police.

"No one's going to call the cops," said Alfredo Gutierrez, a former state Senate majority leader who opposed the bill. He said law-abiding immigrants of all types were fleeing the state out of fear of being subjected to racial profiling.

"They're getting rid of the folks who would report the crooks," Gutierrez said. "The crooks are staying. This is like heaven for them."

John Garcia, a political science professor at the University of Arizona who studies immigration and crime, said that illegal immigrants' disproportionate numbers in the criminal justice system might reflect steps Arizona has long taken to criminalize their presence — steps that have peaked with SB 1070.

In 2006, Maricopa County Sheriff Joe Arpaio interpreted a state law making human smuggling a felony as also making it a crime to be smuggled, and he began prosecuting truckloads of illegal immigrants being transported through the state. Arpaio has made catching illegal immigrants a priority.

Gutierrez said he was not surprised to find a greater proportion of illegal immigrants committing crimes. "There's a greater mix of bad folks coming up who don't care if they're caught," he said. And as more law-abiding immigrants fear talking to authorities, the state will become more welcoming to criminals, he said.

Of the lawmakers who warn of illegal immigrants committing more and more crimes, Gutierrez said, "They're creating a self-fulfilling prophecy."